firmware: zynqmp: Clean up zynqmp_power_probe

firmware: zynqmp: Clean up zynqmp_power_probe

Fix error debug messages to be more accurate and aligned with debug message
style in the whole file.
And do not initialize ret variable because it is initialized later on and
it is just additional step.

Fixes: 7dddd1a1c9e3 ("firmware: zynqmp: Add zynqmp-power support")
Reported-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>